SES-1803 : Directly call constructor in NativeLiteral
SES-1803 : Explicitly set datatype for language literals in LiteralImpl
SES-1803 : Enforce xsd:string and rdf:langString in RDFParserHelper
SES-1803 : Explicitly use XSD:String in RDFParserHelper
SES-1803 : Rework LiteralImpl a little